Overpotential indicator device control circuit



June 10, 1969 1A. S TEWA RT 3,449,737

OVERIQTENTIAI; lNDICATOR DEVlCE CONTROL CIRCUIT Filed April 1, 1965 United States Patent OVERPOTENTIAL INDICATOR DEVICE CONTROL CIRCUIT John A. Stewart, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Apr. 1, 1965, Ser. No. 444,635 Int. Cl. G08b 21/00; Gf 1/40, 1/52 US. Cl. 340248 3 Claims ABSTRACT OF THE DISCLOSURE A control circuit for an overpotential indicator device having two input terminals across which the potential to be monitored is impressed. A variable reference potential, which varies with potential magnitude applied across the input terminals, and a substantially constant reference potential are applied across the base-emitter electrodes, respectively, of a control transistor. The emittercollector electrodes of a switching transistor are connected in series with the filament of an electric indicating lamp across the input terminals and the base electrode is connected to the collector electrode of the control transistor. With potentials exceeding a predetermined magnitude, the control transistor conducts to provide base-emitter and, consequently, emitter-collector current flow through the switching transistor to complete an energizing circuit for the lamp. To preheat the lamp filament and to produce a faster switching action of the control transistor, the junction between the indicating lamp and the switching transistor is connected to the positive polarity input terminal through a resistor and to the base electrode of the control transistor through the series combination of a resistor and a diode.

This invention relates to overpotential indicator device control circuits and, more specifically, to circuits of this type which are responsive to a predetermined relationship between a constant reference potential and a variable reference potential which is proportional to the supply potential being monitored.

Frequently, it is desirable to continuously monitor a supply potential source and provide for an indication or warning in the event the potential of the monitored source approaches or reaches dangerous or intolerable magnitudes.

With many prior art circuits which were developed for the purpose of providing control for an indicating or warning device, potential sensitive circuit components of the type characterized by the ability to become suddenly highly conductive with applied potentials exceeding a predetermined magnitude were employed and were connected across the monitored source. Although these circuits are satisfactory in operation, it is necessary that the circuit components used have a potential rating of the same order of magnitude as the potential of the supply source being monitored. As the cost of these components increases with the potential ratings, these circuits were quite expensive when used with higher supply potentials.

An overpotential indicator device control circuit which employs components of a potential rating substantially less than that of the source being monitored would substantially reduce the cost.

It is, therefore, an object of this invention to provide an improved overpotential indicator device control circuit.

It is another object of this invention to provide an improved overpotential indicator device control circuit which is responsive to a predetermined relationship between a constant reference potential and a variable referice ence potential which is proportional to the supply potential being monitored.

In accordance with this invention, an overpotential indicating device control circuit is provided wherein a potential sensitive circuit, responsive to a predetermined relationship between a constant reference potential and a variable reference potential which is proportional to the supply potential being monitored, produces a control potential with supply overpotential conditions which triggets a controllable switching device to conduction for completing an energizing circuit for an indicating device.

For a better understanding of the present invention together with additional objects, advantages and features thereof, reference is made to the following description and accompanying drawing in which:

FIGURE 1 schematically sets forth the overpotential indicating device control circuit of this invention, and

FIGURE 2 is a schematic diagram similar to FIGURE 1 with additional optional features.

Referring to FIGURE 1, which schematically sets forth the novel control circuit of this invention, a supply potential, not shown, to be monitored may be impressed across input terminals 11 and 13 and poled as shown.

The energizing circuit for the controlled indicator device is comprised of a controllable switching device, shown as a type PNP transistor 20 having the usual base, emitter and collector electrodes, and an indicator device, herein shown as an incandescent lamp 22, connected in series across input terminals 11 and 13 through lines 34 and 36. To complete the energizing circuit for indicator device 22, therefore, transistor 20 must be triggered to conduction. It is to be specifically understood, however, that other controllable switching devices and other indicating devices which produce visible or audible signals may be employed without departing from the spirit of this invention.

To produce a constant reference potential, the series combination of a fixed resistor 14 and a Zener diode 12 is also connected across input terminals 11 and 13 through lines 34 and 36. The potential appearing at junction 15 is of a positive polarity relative to line 36 and remains substantially constant at a magnitude as determined by the rated breakdown potential of Zener diode 12. It is to be specifically understood, however, that other circuit elements possessing electrical characteristics similar to that of Zener diodes may be substituted therefor without departing from the spirit of the invention.

To produce a variable reference potential which is proportional to and increases and decreases with the monitored supply potential impressed across terminals 11 and 13, the series combination of fixed resistors 16 and 18 are connected across the input terminals 11 and 13. Fixed resistors 16 and 18 provide a simple voltage divider from which the variable reference potential appearing at junction 17 may be taken. The potential appearing at junction 17 is a fraction of the monitored supply potential and increases and decreases in magnitude with increases and decreases of the supply potential magnitude. Therefore, this variable reference potential is proportional to the monitored supply potential.

To produce a control potential for triggering switching transistor 20 to conduction with supply overpotential conditions, as determined by the tolerable maximum magnitude, in response to a predetermined relationship between the constant reference potential appearing at junction 15 and the variable reference potential appearing at junction 17, a type NPN transistor 10 having the usual base, emitter and collector electrodes may be employed.

With transistor 10 connected as shown in FIGURE 1, the predetermined relationship between the constant reference potential and the variable reference potential to which transistor is responsive is when the variable reference potential appearing at junction 17 is slightly more positive than the constant reference potential appearing at junction 15. It is to be specifically understood, however, that this invention is not to be limited to this specific relationship as other relationships may be more satisfactory with certain other applications.

With the monitored supply potential impressed across input terminals 11 and 13, type :NPN transistor 10 is normally not conducting [as resistors 16 and 18 of the voltage divider network are selected to produce a variable reference potential at junction 17 which is of a positive polarity but of a magnitude lower than the constant positive polarity reference potential across Zener diode 12 which appears at junction 15 with supply potentials less than the tolerable maximum. As this baseemitter bias condition does not produce base-emitter current tflow through type NPN transistor '10, this transistor is normally not conducting.

Type PNP transistor is also normally not conducting since its base and emitter electrodes are connected to the same potential with transistor 10 not conducting. The emitter electrode is connected directly to input terminal 11 while the base electrode is connected to input terminal 11 through fixed resistors 24 and 26. That is, a circuit to negative potential line 26 is not established with transistor 10 not conducting. As this base-emitter bias condition does not produce base-emitter current flow through type PNP transistor 20, this transistor is normally not conducting.

With an increase of the monitored supply potential impressed across input terminals 11 and 13, the variable [reference potential appearing at junction 17 also increases in magnitude. The ohmic values of resistors 16 and 18 are selected to produce a reference potential at junction 17 which is slightly more positive than the fixed reference potential at junction 15 when the monitored supply potential increases to a magnitude above the tolerable maximum. This condition is the correct bias requirement to produce base-emitter current flow through a type NPN transistor and transistor 10 conducts. With transistor 10 conducting, a control potential of a positive polarity substantially equal to the constant reference potential appearing at junction 15 appears at junction 19 and is applied to the base electrode of switching transistor 20 through fixed resistor 24, which is included in this circuit to limit the base current of transistor 20- to safe levels.

As this control potential is less positive or more negative than that applied to the emitter of transistor 20, connected directly to positive input terminal 11, the baseemitter bias requirements which permits "base-emitter current flow through a type PNP transistor is established and transistor 20 is biased to conduction.

With transistor 20 conducting, the energizing circuit for indicating device 22 is completed. This energized device, of course, indicates that the potential impressed across input terminals 11 and 13 has exceeded the tolerable maximum magnitude selected.

To improve the operation of this circuit, resistor 28, connected as shown in FIGURE 2, wherein like elements have lbeen given like characters of reference, between positive input terminal 11 and indicating device 22 may be provided. With this connection, resistor 28, selected to be of an ohmic value of sufficient magnitude to prevent full energization of device 22, serves as an in-rush current limiter to this device and, should this device be an incandescent lamp, serves to supply a bleeder current which will preheat the filament without heating it to incandescence, thereby providing for more rapid operation of this lamp with the conduction of transistor 20.

Diode 30 and resistor 32 along with resistor 28 provide a circuit from positive input terminal 11 to the base electrode of transistor 10 which will force a sharper 4 switching action in this transistor as this slightly positive polarity potential aids the variable reference potential which appears at junction 17.

As this novel control circuit is responsive to a predetermined relationship between constant and variable reference potentials rather than absolute potential magnitudes, fractions of the monitored potential may be used to establish this relationship. Therefore, circuit elements of a potential rating which is a fraction of the monitored supply potential may be used.

What is claimed is as follows:

1. An overpotential indicator device control circuit comprising, at least two input terminals across which a supply potential to be monitored is impressed, an indi cat-ing device, a controllable switching device having at least a control electrode, means for connecting said indicating device and said switching device in series across said input terminals, means for producing a constant reference potential, means for producing a variable reference potential which increases and decreases with the potential across said input terminals, a semiconductor device having two input electrodes and an output electrode, means for applying said constant reference potential and said variable reference potential across said input electrodes in such a manner that said semiconductor device conducts when said variable reference potential exceeds said constant reference potential for producing .a control potential with overpotential conditions, a first resistor, means for connecting said firs-t resistor between a selected one of said input terminals and the junction between said indicating device and said switching device, a diode, a second resistor, means for connecting said diode and said second resistor in series between the junction between said indicating device and said switching device and one of said input electrodes of said semiconductor device and means for applying said control potential to the control electrode of said switching device for triggering said switching device to conduction thereby completing an energizing circuit for said indicating device.

2. An overpotential indicator device control circuit comprising, at least two input terminals across which a supply potential to be monitored is impressed, an indicating device, a controllable switching device having at least a control electrode, means for connecting said indicating device and said switching device in series across said input terminals, means for producing a constant reference potential, means for producing a variable reference potential whic-h'increases and decreases with the potential across said input terminals, a transistor device having base, emitter and collector electrodes, means for applying said constant reference potential and said variable rcference potential across said base and emitter electrodes whereby said transistor device conducts when said variable reference potential exceeds said constant reference potential for producing a control potential with overpotential conditions, a first resistor, means for connecting said first resistor between a selected one of said input terminals and the junction between said indicating device and said switching device, a diode, a second resistor, means for connecting said diode and said second resistor in series between the junction between said indicating device and said switching device and said base electrode of said transistor device and means for applying said control potential to the control electrode of said switching device for triggering said switching device to conduction thereby completing an energizing circuit for said indicating device.

3. An overpotential indicator device control circuit comprising, a positive polarity input terminal and a negative polarity input terminal across which a supply potential to be monitored is impressed, an indicating device, a controllable switching device having at least a control electrode, means for connecting said indicating device and said switching device in series across said input terminals, means for producing 'a constant reference potential, means for producing a variable reference potential which increases and decreases with the potential across said input terminals, a transistor device having base, emitter and collector electrodes, means for applying said constant reference potential and said variable reference potential across said base and emitter electrodes whereby said transistor device conducts when said variable reference potential exceeds said constant reference potential for producing a control potential with overpotent-ial conditions, a first resistor, means for connecting said first resistor between said positive input terminal and the junction between said indicating device and said switching device, a diode, a second resistor, means for connecting said diode and said 'second resistor in series between the junction between said indicating device and said switching device and said base electrode of said tran- UNITED STATES PATENTS 2,693,572 11/1954 Chase.

3,139,562 6/1964 Freeborn 31-'7--l48.5 3,241,029 3 /l966 Slomski 31731 XR 3,262,014 7/1966 Conner 317--13 JOHN w. CALDWELL, Primary Examiner. DANIEL K. MYER, Assistant Examiner.

US. Cl. X.R. 3-2322 P0405 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3:449:73, Dated June 0 1969 Inventor(s) John A. Stewart It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, add the following United States patents under "References Cited":

3, 099, 827 7/1963 Wu 340-248 3, 240, 989 3/1966 Grunwaldt 315-136 )CR 3, 262, 016 7/1966 Martin 317-33 3, 354, 448 11/1967 Brolin 340-248-C SIGNED ANU SEALED OCT 21 1969 (SEAL) Attest:

Edward M. Fletcher, 11'. WILLIAM E. W. Ancating Officer g missioner of Pat-ants 

